looks llike ur babies are around 2-3 weeks. hydro, escpecially DWC, which by the way is the bomb, needs to be under 6, like 5.5 - 5.8. 750 ppm's seem way too much even for a heavy indica, especially if under fluros, under 4 weeks and under a foot
depends-if those are the first set of fans and it stops there-i.e-not to any higher fans-than just back off the ppm's. if has pregoressed than flush your res and lower ppm's to 400. may want to do that anyway.
and brother please lower that ph to under 6